75 



sity due to the formation of new genes or change in old ones. After 

 such long continued and still effective selection reciprocal crosses between 

 high and low lines still give no indication of sex-linkage. The germinal 

 changes of small degree which must be assumed to explain such a long 

 continued effect of selection therefore are not changes in the bar gene 

 nor are they due to new accessory genes occurring in the sex chromosome. 

 New genes must have arisen in the autosomes. Experiments are under 

 way to detei"mine their chromosomal loci more definitely. 



Germinal Changes op Marked Degree. 



In the high facet selection line marked mutations have occurred 

 which have yielded full-eyed individuals indistinguishable from the wild 

 ones which originally mutated to form the bar stock. These new full- 

 eyed flies are genetically of two distinct types. One type is the result 

 of a reverse mutation involving the return of the bar gene to the orig- 

 inal full-eye-producing condition. Its hereditary behavior is similar to 

 that of the wild Drosophila in all the tests that have been made. 



The other type retains the bar gene unchanged, the somatic appea.r- 

 ance of full eye being due to the formation of a modifying gene outside 

 of the sex chromosome. This new gene is effective in producing full eye 

 when present in double dose in females heterozygous for the bar gene. 

 Such full-eyed females when crossed with wild full-eyed males produce 

 males half of whom are bar and females half of whom are heterozygous 

 bar. 



In males with the bar gene and in females homozygous for bar the 

 double dose of the new gene produces an eye which is nearly full but 

 which differs from full in the presence of a defect at the anterior margin. 

 Such an eye may be designated by the term "emarginate." Emarginate 

 females when crossed with full wild males give males all of whom are 

 bar and females all of whom are heterozygous bar. The reciprocal cross 

 gives males all of whom are full and females all of whom are heter- 

 ozygous bar. Numerous tests bear out in detail the hypothesis as stated 

 above indicating that the chromosomal formula for this type of female 



B m B 



with a full eye is ^=^= , for the emarginate-eyed female — — — 



m B 



m B m . 



, and for the emarginate-eyed male . Experiments 



are under way to determine the exact locus of the new gene. 



